Memory system and method using partial ecc to achieve low power refresh and fast access to data

ABSTRACT

A DRAM memory device includes several banks of memory cells each of which are divided into first and second sets of memory cells. The memory cells in the first set can be refreshed at a relatively slow rate to reduce the power consumed by the DRAM device. Error checking and correcting circuitry in the DRAM device corrects any data retention errors in the first set of memory cells caused by the relatively slow refresh rate. The memory cells in the second set are refreshed at a normal rate, which is fast enough that data retention errors do not occur. A mode register in the DRAM device may be programmed to select the size of the second set of memory cells.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.11/546,692, filed Oct. 11, 2006. This application is incorporated byreference herein in its entirety and for all purposes.

TECHNICAL FIELD

This invention relates to dynamic random access memory (“DRAM”) devices,and, more particularly, to a method and system for operating a memorysystem in a low power mode while allowing frequently accessed data to bequickly accessed.

BACKGROUND OF THE INVENTION

As the use of electronic devices, such as personal computers, continuesto increase, it is becoming ever more important to make such devicesportable. The usefulness of portable electronic devices, such asnotebook computers, is limited by the limited length of time batteriesare capable of powering the device before needing to be recharged. Thisproblem has been addressed by attempts to increase battery life andattempts to reduce the rate at which such electronic devices consumepower.

Various techniques have been used to reduce power consumption inelectronic devices, the nature of which often depends upon the type ofpower consuming electronic circuits that are in the device. For example,electronic devices such a notebook computer, typically include dynamicrandom access memory (“DRAM”) devices that consume a substantial amountof power. As the data storage capacity and operating speeds of DRAMdevices continue to increase, the power consumed by such devices hascontinued to increase in a corresponding manner. Therefore, manyattempts to reduce the power consumed by an electronic device havefocused on reducing the power consumption of DRAM devices.

In general, the power consumed by a DRAM device increases with both thecapacity and the operating speed of the DRAM devices. The power consumedby DRAM devices is also affected by their operating mode. A DRAM devicefor example, will generally consume a relatively large amount of powerwhen the memory cells of the DRAM device are being refreshed. As iswell-known in the art, DRAM memory cells, each of which essentiallyconsists of a capacitor, must be periodically refreshed to retain datastored in the DRAM device. Refresh is typically performed by essentiallyreading data bits from the memory cells in each row of a memory cellarray and then writing those same data bits back to the same cells inthe row. A relatively large amount of power is consumed when refreshinga DRAM because rows of memory cells in a memory cell array are beingactuated in the rapid sequence. Each time a row of memory cells isactuated, a pair of digit lines for each memory cell are switched tocomplementary voltages and then equilibrated. As a result, DRAMrefreshes tend to be particularly power-hungry operations. Further,since refreshing memory cells must be accomplished even when the DRAM isnot being used and is thus inactive, the amount of power consumed byrefresh is a critical determinant of the amount of power consumed by theDRAM over an extended period. Thus many attempts to reduce powerconsumption in DRAM devices have focused on reducing the rate at whichpower is consumed during refresh.

Refresh power can, of course, be reduced by reducing the rate at whichthe memory cells in a DRAM are being refreshed. However, reducing therefresh rate increases the risk that data stored in the DRAM memorycells will be lost. More specifically, since, as mentioned above, DRAMmemory cells are essentially capacitors, charge inherently leaks fromthe memory cell capacitors, which can change the value of a data bitstored in the memory cell over time. However, current leaks fromcapacitors at varying rates. Some capacitors are essentiallyshort-circuited and are thus incapable of storing charge indicative of adata bit. These defective memory cells can be detected during productiontesting, and can then be repaired by substituting non-defective memorycells using conventional redundancy circuitry. On the other hand,current leaks from most DRAM memory cells at much slower rates that spana wide range. A DRAM refresh rate is chosen to ensure that all but a fewmemory cells can store data bits without data loss. This refresh rate istypically once every 64 ms. The memory cells that cannot reliably retaindata bits at this refresh rate are detected during production testingand replaced by redundant memory cells.

One technique that has been used to prevent data errors during refreshas well as at other times is to generate an error correcting code “ECC,”which is known as a “syndrome,” from each item of stored data, and thenstore the syndrome along with the data. When the data are read from thememory device, the syndrome is also read, and it is then used todetermine if any bits of the data are in error. As long as not too manydata bits are in error, the syndrome may also be used to correct theread data. Some DRAM devices include a mode register that may be set toselectively operate the DRAM device in either a normal mode or an ECCmode.

The use of ECC techniques can allow DRAM devices to be refreshed at aslower refresh rate since resulting data bit errors can be corrected.The use of a slower refresh rate can provide the significant advantageof reducing the power consumed by DRAM devices. Prior to entering areduced power refresh mode, each item of data is read. A syndromecorresponding to the read data is then generated and stored in the DRAMdevice. When exiting the reduced power refresh mode, the each item ofdata and each corresponding syndrome are read from the DRAM device. Theread syndrome is then used to determine if the item of read data is inerror. If the item of read data is found to be in error, the readsyndrome is used to correct the read item of data, and the incorrectitem of data is then overwritten with the corrected item of data.

The use of the above-described ECC techniques to allow refresh at arelatively low rate can markedly reduce the power consumed by DRAMdevice in many applications, particularly where the DRAM device is notbeing accessed for an extended period. However, if the DRAM device isbeing frequently accessed, the power consumed in reading syndromes anddata, using the read syndromes to check and possibly correct the readdata, and writing any corrected data to the DRAM device can exceed thepower saved by using ECC techniques to refresh at a reduced rate.Moreover, it can require a considerable period of time to exit thereduced power refresh mode when using ECC techniques as described above,thus preventing data stored in the DRAM device from being quicklyaccessed. As a result, there are many applications where a reduced powerrefresh mode using ECC techniques are not practical.

For example, one application in which reduced power consumption is veryimportant, but access to a DRAM device is frequent, is in the field ofcellular telephones. DRAM devices are frequently used in cellulartelephones to store a variety of data, such as paging protocols, textmessages, image data, etc. When the cellular telephone is powered but atelephone call is not currently active, the cellular telephone isessentially inactive. During such periods of inactivity, almost all ofthe data stored in the DRAM device is not being accessed. However, asmall portion of the data stored in the DRAM device must be accessed anytime power is applied to the cellular telephone. For example, datacorresponding to a paging protocol must be accessed to determine if acall is being made to the cellular telephone. The protocol data isaccessed during each paging period which occurs on a periodic basis,such as once every one-half second. During the paging period, thecellular telephone uses the protocol data to transmit a probe, which isreceived by one or more cellular sites that are in range of the cellulartelephone. A cellular site then transmits a message back to the cellulartelephone if an incoming call to the cellular telephone is being made.

The need for at least some data stored in DRAM devices to be frequentlyand immediately available makes it impractical to use the previouslydescribed ECC techniques to reduce power in an extended refresh mode.The use of such techniques would require the DRAM device to enter andexit the reduced power refresh mode every paging period, which, asmentioned above, is on the order of once every one-half second. As aresult, the read data stored in the DRAM device might not be accessiblewhen the data were needed, particularly if the DRAM device contains alarge number of memory cells. Even if the DRAM device could enter andexit the reduced power refresh mode at a sufficient rate, the timerequired to enter and exit the reduced power refresh mode might verywell reduce the duration of the reduced power refresh period to theextent that very little power was saved. As a result, DRAM devices usedin cellular telephones generally are operated with faster refresh ratesthan otherwise needed as a result of the need for the entire device tobecome active so that the protocol data can be accessed every pagingperiod. However, doing so causes the cellular telephones to consumesubstantial power, thereby reducing the useful life of batteriespowering cellular telephones before a recharge is needed.

There is therefore a need for a memory system and method that iseffective in allowing a DRAM device to operate in a reduced powerrefresh mode using ECC techniques, but does so in a manner that does notdelay access to data stored in the DRAM or minimize the benefits ofoperating in the reduced power refresh mode.

SUMMARY OF THE INVENTION

An error checking and correcting semiconductor device and methodperforms a reduced power refresh using ECC techniques only for memorycells that store infrequently accessed data. The memory cells that storethis infrequently accessed data may be refreshed at a relatively lowrate, thereby substantially reducing the power consumed by thesemiconductor device. Memory cells that store frequently accessed datamay be refreshed at a normal rate that does not require ECC techniquesto ensure data integrity. As a result, this frequently accessed data isimmediately available without the need to enter and exit the reducedpower refresh mode. In the event most of the data stored in thesemiconductor device are accessed infrequently, the power saved canapproach the power savings that are achieved by placing the entiresemiconductor device in the reduced power refresh mode. When used in acellular telephone, most of the memory cells in the semiconductor deviceare refreshed in the reduced power refresh mode since the data storedtherein are only accessed when a call is received. Data corresponding tothe paging protocol, which must be accessed each paging period, arestored in memory cells that are refreshed at the normal rate. As aresult, the protocol data is immediately available, and power is notconsumed every paging period by entering and exiting the reduced powerrefresh mode for such data.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a computer system according to oneembodiment of the invention.

FIG. 2 is a block diagram of a memory device according to one embodimentof the invention that may be used in the computer system of FIG. 1.

FIG. 3 is a memory map showing the logical ordering of address space inthe memory device of FIG. 2.

FIG. 4 is a block diagram of a cellular telephone that uses the memorydevice of FIG. 2.

DETAILED DESCRIPTION

A computer system 100 according to one embodiment of the invention isshown in FIG. 1. The computer system 100 includes a central processorunit (“CPU”) 14 coupled to a system controller 16 through a processorbus 18. The system controller 16 is coupled to input/output (“I/O”)devices (not shown) through a peripheral bus 20 and to an I/O controller24 through an expansion bus 26. The I/O controller 24 is also connectedto various peripheral devices (not shown) through another I/O bus 28.

The system controller 16 includes a memory controller 30 that is coupledto a dynamic random access memory (“DRAM”) device 102 through an addressbus 36, a control bus 38, and a data bus 42. The locations in the DRAMdevice 102 to which data are written and data are read are designated byaddresses coupled to the DRAM device 102 on the address bus 36. Theoperation of the DRAM device 102 is controlled by control signalscoupled to the DRAM device 102 on the control bus 38.

In other embodiments of the invention, the memory controller 30 may becoupled to several DRAM devices or to one or more memory modules (notshown) through the address bus 36, the control bus 38, and the data bus42. Each of the memory modules would normally contain several of theDRAM devices 102.

With further reference to FIG. 1, the DRAM device 102 includes a moderegister 44, a syndrome memory 120, a DRAM array 34, and ECC logic 110.The ECC logic 110 generates a syndrome from write data received from thememory controller 30, and stores the syndrome in the syndrome memory 120while the write data are being stored in the DRAM array 34. When dataare read from the DRAM device 102, the read data are coupled from DRAMarray 34 to the ECC logic 110, and the syndrome is coupled from thesyndrome memory 120 to the ECC logic 110. The ECC logic 110 then usesthe syndrome to determine if the read data contains an erroneous databit, and, if more than one data bit is not in error, to correct theerroneous data bit. The corrected read data are then coupled to thememory controller 30 through the data bus 42. Although the syndromememory 120 may be a separate memory array in the DRAM device 102 asshown in FIG. 1, it may alternatively be included in the same array ofDRAM cells that are used to store data, as explained in greater detailbelow. As explained above, the use of ECC allows the refresh rate of thememory cells in the DRAM array 34 to be reduced to a rate at which somedata retention errors can occur since such errors can be corrected usingthe syndromes stored in the syndrome memory 120 and the ECC logic 110.

As explained in greater detail below, the mode register 44 in the DRAMdevice 102 includes one or more bits that can be set to partition aregion 124 of the DRAM array 34 of various sizes for exclusion fromoperation in the low power, reduced refresh rate mode using ECCtechniques. The partitioned memory region 124 is refreshed at the normalrefresh rate that is selected to ensure that data retention errors donot occur. As a result, the data read from the DRAM array 34 in thepartitioned region 124 is immediately available to the CPU 14 throughthe memory controller 30 without the need to first read a correspondingECC syndrome and then use the syndrome to check and possibly correctdata read from the DRAM array 34. The area of the DRAM array 34 not inthe partitioned memory region 124 can be refreshed at a reduced rateusing ECC techniques to correct any data errors resulting from thereduced refresh rate. In the event the partitioned memory region 124 isa relatively small portion of the DRAM array 34, the reduction in powerconsumption resulting from refreshing the remainder of the DRAM array 34at the reduced rate can approach the power savings resulting fromoperating the entire DRAM array 34 at the reduced refresh rate using ECCtechniques.

Although the memory system 100 shown in FIG. 1 uses the mode register 44in the DRAM device 102 to select whether the DRAM array 34 will bepartitioned as described above, and, if so, the size of the partitionedregion 124, it should be understood that other means may be used. Forexample, data could be written to the DRAM array 34 itself thatspecifies whether the DRAM array 34 should be partitioned, and, if so,the size of the partitioned region 124. Other techniques may also beused.

In operation, prior to the DRAM device 102 entering a low power refreshmode, the DRAM device performs a read and syndrome generating operationfor all regions of the DRAM array 34 that is not in the partitionedregion 124. More specifically, the mode register 44 is first accessed todetermine if a region of the DRAM array 34 is to be partitioned, and, ifso the size of the partitioned region 124. The ECC logic 110 is thenenabled by suitable means, such as by coupling a command signal to theDRAM device 102 through the memory controller 30 and control bus 38 thatenables a control register in the DRAM device 102. However, the CPU 14may enable the ECC logic 110 by other means, such as by coupling controlsignals directly to the ECC logic 110, by coupling an unsupportedcommand to the DRAM device 102, use of a specific sequence ofoperations, or by other means. In any case, once the ECC logic 110 hasbeen enabled, the CPU 14 performs a read operation to the regions of thememory array 34 that are outside of the partitioned region 124. The readoperation is preferably performed in a burst read mode to minimize thetime required for the read operation. During the read operation, theDRAM device 102 generates syndromes from the read data, and stores thesyndromes in the syndrome memory 120. The DRAM device 102 then enters alow power refresh mode in which the memory cells in the array 34 outsideof the partitioned region 124 are refreshed at a rate that issufficiently low that data retention errors may occur. This rate ispreferably at least twice as slow as the rate at which memory cells inthe partitioned region 124 of the array 34 are refreshed. The memorycells in the partitioned region 124 of the array 34 are refreshed at anormal rate that is generally sufficient for no data retention errors tooccur. In one embodiment of the invention, the CPU 14 leaves the ECClogic 110 enabled during the low power refresh mode to correct any dataretention errors as they occur. In another embodiment of the invention,the CPU 14 disables the ECC logic 110 after all of the syndromes havebeen stored and before entering the low power refresh mode. In thisembodiment, the CPU 14 corrects any data retention errors that haveoccurred when exiting the low power refresh mode.

When exiting the low power refresh mode, the DRAM device 102 performs aread and correct operation for all regions of the DRAM array 34 that arenot in the partitioned region 124. More specifically, the CPU 14 enablesthe ECC logic 110 if it was not enabled during the refresh mode. The CPU14 then reads data from the memory array 34 outside of the partitionedregion 124, again preferably using a burst read mode. During these readoperations, the ECC logic 110 receives the read data from the memoryarray 34 and the corresponding syndromes from the syndrome memory 120.The ECC logic 110 then uses the syndromes to check the read data and tocorrect any errors that are found. The ECC logic 110 then writes thecorrected data to the memory array 34. Once the regions of the memoryarray 34 outside of the partitioned region 124 have been read, therefresh rate for that region is increased to the normal refresh ratethat has been used for the partitioned region 124. The CPU 14 candisable the ECC logic 110.

In other embodiments of the invention, the CPU 14 initiates a readoperation prior to entering the low power refresh mode, but the actualreading of data from the protected areas is accomplished by sequencerlogic (not shown) in the DRAM device 102 or in a memory modulecontaining DRAMdevice 102. The operation of the sequencer logic could beinitiated by commands from the CPU 14 other than a read command, such asby issuing commands for a “dummy” operation, i.e., an operation that isnot actually implemented by the DRAM device 34.

In still another embodiment of the invention, the data stored in memoryarray 34 outside of the partitioned region 124 are not checked andcorrected when exiting the low power refresh mode. Instead, the ECC moderemains active during normal operation, and the data stored in thememory array 34 outside of the partitioned region 124 are checked usingthe stored syndromes whenever that data are read during normaloperation. This embodiment requires that the syndrome memory 120 remainpowered during normal operation, at least until all of the data storedin the memory array 34 outside of the partitioned region 124 have beenread. Other techniques may also be used.

A synchronous DRAM (“SDRAM”) 200 according to one embodiment of theinvention is shown in FIG. 2. The SDRAM 200 includes an address register212 that receives bank addresses, row addresses and column addresses onan address bus 214. The address bus 214 is generally coupled to a memorycontroller like the memory controller 30 shown in FIG. 1. Typically, abank address is received by the address register 212 and is coupled tobank control logic 216 that generates bank control signals, which aredescribed further below. The bank address is normally coupled to theSDRAM 200 along with a row address. The row address is received by theaddress register 212 and applied to a row address multiplexer 218. Therow address multiplexer 218 couples the row address to row address latch& decoder circuit 220 a-d for each of several banks of memory cellarrays 222 a-d, respectively.

One of the latch & decoder circuits 220 a-d is enabled by a controlsignal from the bank control logic 216 depending on which bank of memorycell arrays 222 a-d is selected by the bank address. The selected latch& decoder circuit 220 applies various signals to its respective bank 222as a function of the row address stored in the latch & decoder circuit220. These signals include word line voltages that activate respectiverows of memory cells in the banks 222 a-d.

The row address multiplexer 218 also couples row addresses to the rowaddress latch & decoder circuits 220 a-d for the purpose of refreshingthe memory cells in the banks 222 a-d. The row addresses are generatedfor refresh purposes by a pair of refresh counters 230, 232. Duringoperation in the low power, reduced refresh rate mode described above,the refresh counter 230 periodically increments to output row addressesfor rows in the banks 222 a-d outside of a partitioned region of one ormore of the banks 222 a-d. The refresh counter 230 causes the memorycells in the banks 222 a-d outside of a partitioned region to berefreshed at a rate that is sufficiently low that data errors are likelyto occur. The refresh of the memory cells in the banks 222 a-d outsideof the partitioned region may be performed at intervals as long as 1 to3 seconds depending on the design and fabrication of the SDRAM 200.Refreshing the memory cells at this low rate causes relatively littlepower to be consumed during self-refresh. The refresh counter 232periodically increments to output row addresses for rows in thepartitioned region in one or more of the banks 222 a-d at a normalrefresh rate that generally does not result in data retention errors.The refresh of the memory cells in the partitioned region is typicallyperformed every 64 ms.

After the bank and row addresses have been applied to the addressregister 212, a column address is applied to the address register 212.The address register 212 couples the column address to a column addresscounter/latch circuit 234. The counter/latch circuit 234 stores thecolumn address, and, when operating in a burst mode, generates columnaddresses that increment from the received column address. In eithercase, either the stored column address or incrementally increasingcolumn addresses are coupled to column address & decoders 238 a-d forthe respective banks 222 a-d. The column address & decoders 238 a-dapply various signals to respective sense amplifiers 240 a-d throughcolumn interface circuitry 244. The column interface circuitry 244includes conventional I/O gating circuits, DQM mask logic, read datalatches for storing read data from the memory cells in the banks 222 a-dand write drivers for coupling write data to the memory cells in thebanks 222 a-d.

The column interface circuitry 244 also includes an ECCgenerator/checker 246 that essentially performs the same function as theECC logic 110 in the DRAM 102 of FIG. 1. The ECC generator/checker 246may be implemented by conventional means, such as by chains of exclusiveOR gates implementing a Hamming code. Syndromes corresponding to thedata stored in the memory cells in the banks 222 a-d outside of thepartitioned region may be stored in one or more of the banks 222 a-d.When data are read from the memory cells of the banks 222 a-d outside ofthe partitioned region, the corresponding syndrome is also read and thencoupled to the ECC generator/checker 246. Data read from one of thebanks 222 a-d outside the partitioned region are sensed by therespective set of sense amplifiers 240 a-d and then checked and, ifnecessary, corrected, by the ECC generator/checker 246. The data arethen coupled to a data output register 248, which applies the read datato a data bus 250. Data read from one of the banks 222 a-d in thepartitioned region are sensed by the respective set of sense amplifiers240 a-d and then coupled to the data bus 250 through the data outputregister 248 without being processed by the ECC generator/checker 246.

Data to be written to the memory cells in one of the banks 222 a-doutside of the partitioned region are coupled from the data bus 250through a data input register 252 to the ECC generator/checker 246,which generates a corresponding syndrome. The write data and thecorresponding syndrome are then coupled to write drivers in the columninterface circuitry 244, which couple the data and syndrome to thememory cells in one of the banks 222 a-d. Data to be written to thememory cells in the partitioned region of one or more of the banks 222a-d are coupled from the data bus 250 through a data input register 252directly to the write drivers in the column interface circuitry 244without interfacing with the ECC generator/checker 246. A data masksignal “DQM” may be applied to the column interface circuitry 244 andthe data output register 248 to selectively alter the flow of data intoand out of the column interface circuitry 244, such as by selectivelymasking data to be read from the banks of memory cell arrays 222 a-d.

The above-described operation of the SDRAM 200 is controlled by controllogic 256, which includes a command decoder 258 that receives commandsignals through a command bus 260. These high level command signals,which are typically generated by a memory controller such as the memorycontroller 30 of FIG. 1, are a clock a chip select signal CS#, a writeenable signal WE#, a column address strobe signal CAS#, and a rowaddress strobe signal RAS#, with the “#” designating the signal asactive low. Various combinations of these signals are registered asrespective commands, such as a read command or a write command. Thecontrol logic 256 also receives a clock signal CLK and a clock enablesignal CKE, which allow the SDRAM 200 to operate in a synchronousmanner. The control logic 256 generates a sequence of control signalsresponsive to the command signals to carry out the function (e.g., aread or a write) designated by each of the command signals. The controllogic 256 also applies signals to the refresh counter 230 to control theoperation of the refresh counter 230 during refresh of the memory cellsin the banks 222. The control signals generated by the control logic256, and the manner in which they accomplish their respective functions,are conventional. Therefore, in the interest of brevity, a furtherexplanation of these control signals will be omitted.

The control logic 256 also includes a mode register 264 that may beprogrammed by signals coupled through the command bus 260 duringinitialization of the SDRAM 200. The mode register 264 then generatesmode control signals that are used by the control logic 256 to controlthe operation of the SDRAM 200 in various modes. One or more bits of themode register 264 are refresh mode bits that, when set, causes the SDRAM200 to partition the banks 222 a-d for operation in a normal refreshmode as described above while the remainder of the banks 222 a-d operatein a low power, reduced refresh rate mode using ECC techniques.

Finally, the control logic 256 also includes an ECC controller 270 thatcauses the control logic 256 to issue control signals to the ECCgenerator/checker 246 and other components to generate syndromes forstorage in the banks 222 a-d, and to check and correct data read fromthe banks 222 a-d outside the partitioned region using the storedsyndromes. The ECC controller 270 is enabled by signals from the moderegister 264 to control the operation of the SDRAM 200 in the low power,reduced refresh rate mode. If a single bit in the mode register 264 isused, the mode register simply enables or disables the use of a fixedpartition for refresh at the normal rate and the remainder of the banks222 a-d for refresh at the reduced rate using ECC techniques. Ifmultiple bits of the mode register 264 are used, one bit enables ordisables the low power, reduced refresh mode, and the remaining bits areused to specify the size of the partitioned region of the banks 222 a-dthat will be refreshed at the normal rate. For example, if two bits ofthe mode register 264 are used, the bits might be decoded as follows:

-   -   “00”—Normal Mode    -   “01”—Low Power Mode with 1 Mb partition    -   “10”—Low Power Mode with 2 Mb partition    -   “11”—Low Power Mode with 4 Mb partition.        Other arrangements may also be used. Furthermore, as pointed out        with respect to the computer system 100 of FIG. 1, other        techniques not involving the mode register 264 may be used to        enable or disable the low power refresh mode and to set the size        of a partitioned region of memory that will be refreshed at the        normal rate.

Although the SDRAM device 200 can have a variety of configurations, inone embodiment the address space of the SDRAM device 200 is logicallyorganized as shown in FIG. 3, although the physical configuration of theSDRAM device 200 will be somewhat different. As shown in FIG. 3, eachrow contains 128 column groups, and each column group contains 128 bitsof data arranged as 8 16-bit words plus an additional 8 bits that areused to store the ECC syndrome. Therefore, each logical row includes17,408 bits of data, i.e., the product of the 128 data bits plus 8 ECCbits in each column group and the 128 column groups in each row. In oneembodiment of the SDRAM device 200 having 64 rows in each bank 222 a-d,each row contains 1,114,112 memory cells, i.e., the product of 17,408memory cells in each row and 64 rows. Insofar as each column groupcontains 136 bits, i.e., 128 data bits and 8 ECC bits, there are 8,192column groups in each of the banks 222 a-d, i.e. 1,114,112 memory cellsdivided by 136 bits. Each of these 8,192 column groups can bepartitioned for refresh in the low power, reduced refresh rate mode. Inone embodiment, the banks 222 a-d are partitioned as follows:

Bank 222 a

-   -   Partition 1—8192 rows×128 columns (8 bits each)=1 Mb    -   Partition 2—8192 rows×128 columns (8 bits each)=1 Mb    -   Partition 3—16384 rows×128 columns (8 bits each)=2 Mb    -   Non-Partitioned—1015808 rows×128 columns (8 bits each)=124 Mb

Bank 222 b

-   -   Non-Partitioned—1048576 rows×128 columns (8 bits each)=128 Mb

Bank 222 c

-   -   Non-Partitioned—1048576 rows×128 columns (8 bits each)=128 Mb

Bank 222 d

-   -   Non-Partitioned—1048576 rows×128 columns (8 bits each)=128 Mb

In another embodiment of the invention, the partitioned in the banks 222a-d are implemented in a symmetrical manner, as follows:

Bank 222 a

-   -   Partition 1—8192 rows×128 columns (8 bits each)=1 Mb    -   Partition 2—8192 rows×128 columns (8 bits each)=1 Mb    -   Partition 3—16384 rows×128 columns (8 bits each)=2 Mb    -   Non-Partitioned—1015808 rows×128 columns (8 bits each)=124 Mb

Bank 222 b

-   -   Partition 1—8192 rows×128 columns (8 bits each)=1 Mb    -   Partition 2—8192 rows×128 columns (8 bits each)=1 Mb    -   Partition 3—16384 rows×128 columns (8 bits each)=2 Mb    -   Non-Partitioned—1015808 rows×128 columns (8 bits each)=124 Mb

Bank 222 c

-   -   Partition 1—8192 rows×128 columns (8 bits each)=1 Mb    -   Partition 2—8192 rows×128 columns (8 bits each)=1 Mb    -   Partition 3—16384 rows×128 columns (8 bits each)=2 Mb    -   Non-Partitioned—1015808 rows×128 columns (8 bits each)=124 Mb

Bank 222 d

-   -   Partition 1—8192 rows×128 columns (8 bits each)=1 Mb    -   Partition 2—8192 rows×128 columns (8 bits each)=1 Mb    -   Partition 3—16384 rows×128 columns (8 bits each)=2 Mb    -   Non-Partitioned—1015808 rows×128 columns (8 bits each)=124 Mb        This symmetrical partitioning has the advantage of allowing        multi-bank operation during the paging operation. Other        partitioning arrangements can, of course, be used.

FIG. 4 is a block diagram of a cellular telephone 300 according to oneembodiment of the invention. The cellular telephone 300 includescellular telephone electronics 310 of conventional design, whichnormally access a memory device to store paging protocols and otherinformation. However, the cellular telephone electronics 310 shown inFIG. 4 accesses the SDRAM 200 shown in FIG. 2. The cellular telephoneelectronics 310 initially program the mode register 264 with a mode bitto enable the low power, reduced refresh rate using ECC techniques, asdescribed above. The cellular telephone electronics 310 also program themode register 264 with one or more bits to set the size of thepartitioned region in the banks 222 a-d of memory cells. The size of thepartitioned region to store paging protocols and any other frequentlyaccessed data is typically 1-2 Mb. The cellular telephone 300 includesuser interface devices 320 coupled to the cellular telephone electronics310 to allow a user to make telephone calls, provide and receiveinformation, take photographs, etc. The cellular telephone electronics310 typically includes at least a keyboard, a display, a microphone, andan earphone. The cellular telephone 300 also includes a battery 330connected to the cellular telephone electronics 310 and the SDRAM 200 tosupply operating power. Because of the relatively low power consumed bythe SDRAM 200 using the low power, reduced refresh rate mode, thebattery 330 has a relatively long operating life between recharges.

Although the present invention has been described with reference to thedisclosed embodiments, persons skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention. For example, although the reduced powerrefresh mode has been described in the context of a self-refresh reducedpower mode, it will be understood that it may also be used in otherrefresh modes. Other variations will also be apparent to one skilled inthe art. Accordingly, the invention is not limited except as by theappended claims.

1. A method of refreshing an array of memory cells arranged in rows and columns in a dynamic random access memory (“DRAM”) device, the method comprising: refreshing a first set of the memory cells in the array at a first rate; refreshing a second set of the memory cells in the array at a second rate that is significantly faster than the first rate; and using ECC technique to correct any data retention errors that arise in the memory cells in the first set.
 2. The method of claim 1 wherein the act of using ECC technique to correct any data retention errors that arise in the memory cells in the first set comprises: when writing data to the memory cells in the first set, generating a syndrome corresponding to the data and storing the syndrome; and when reading data from the memory cells in the first set, reading the stored syndrome corresponding to the data and using the syndrome to correct any errors that exist in the data.
 3. The method of claim 2, further comprising overwriting the read data stored in the memory cells in the first set with data that has been corrected using the corresponding syndrome.
 4. The method of claim 2 wherein the act of storing the syndrome comprises storing the syndrome in the SDRAM device.
 5. The method of claim 4 wherein the act of storing the syndrome in the SDRAM device comprises storing the syndrome in memory cells in the first set.
 6. The method of claim 1, further comprising selectively enabling the first set of the memory cells in the array to be refreshed at the first rate.
 7. The method of claim 6 wherein the DRAM device includes a mode register that is programmable to select various operating modes of the DRAM device, and wherein the act of enabling the first set of the memory cells in the array to be refreshed at the first rate comprises programming the mode register to enable the first set of the memory cells in the array to be refreshed at the first rate.
 8. The method of claim 1, further comprising selecting the size of the second set of the memory cells in the array.
 9. The method of claim 8 wherein the DRAM device includes a mode register that is programmable to select various operating modes of the DRAM device, and wherein the act of selecting the size of the second set of the memory cells in the array comprises programming the mode register with at least one bit having values which correspond to respective sizes of the second set of the memory cells in the array.
 10. The method of claim 1 wherein the memory cells in the first and second sets comprise all of the memory cells in the DRAM device.
 11. A memory device, comprising: an array of memory cells arranged in rows and columns; an address decoder receiving row addresses and column addresses, the address decoder being operable to activate a row of memory cells corresponding to each received row address and to select a memory cell in a column of memory cells corresponding to each received column address; a read data path operable to couple read data from selected memory cells in an activated row to a plurality of data bus terminals; a write data path operable to couple write data from the plurality of data bus terminals to selected memory cells in an activated row; refresh circuitry operable to refresh a first set of the memory cells in the array at a first rate, the refresh circuitry further being operable to refresh a second set of the memory cells in the array at a second rate that is significantly faster than the first rate; control logic operable to cause the write data to be coupled from the data bus terminals to the array of memory cells and to cause the read data to be coupled from the array of memory cells to the data bus terminals; and an error checking and correcting system coupled to the array of memory cells and being operable to correct any data retention errors that arise in the memory cells in the first set.
 12. The memory device of claim 11 wherein the error checking and correcting system comprises error checking and correcting logic coupled to the read data path and the write data path, the error checking and correcting logic being operable to generate an error checking and correcting syndrome from data written to the first set of memory cells, the error checking and correcting logic further being operable to use the syndrome to check and correct data read from the first set of memory cells.
 13. The memory device of claim 12 wherein the control logic is operable to cause the generated syndromes to be written to the array of memory cells when the corresponding data are written to the array of memory cells in the first set, the control logic further being operable to cause the generated syndromes to be read from the array of memory cells and coupled to the error checking and correcting logic when the corresponding data are read from the array of memory cells in the first set.
 14. The memory device of claim 11 wherein the reduced power refresh mode comprises a self-refresh mode.
 15. The memory device of claim 11 wherein the control logic further comprises a mode register that includes refresh mode bits that can be set to cause the memory cells in the first set to be refreshed at either the first rate or the second rate.
 16. The memory device of claim 11 wherein the refresh circuitry further comprises an error checking and correcting system that is operable to generate a syndrome corresponding to data written to the first set of memory cells and to store the syndrome in the array of memory cells, the error checking and correcting system further being operable to read the stored syndrome corresponding to the data being read from the memory cells in the first set, and to use the syndrome to correct any errors that exist in the data.
 17. The memory device of claim 11 wherein the refresh circuitry comprises a first refresh counter operable to generate row addresses for the first set of memory cells, and a second refresh counter operable to generate row addresses for the second set of memory cells, the second refresh counter being incremented at a rate that is faster than the rate at which the first refresh counter is incremented.
 18. The memory device of claim 11 wherein the control logic further comprises a mode register that includes refresh mode bits that can be programmed to select the size of the second set of the memory cells in the array.
 19. The memory device of claim 11 wherein the memory cells in the array comprise dynamic random access memory cells. 